Method of sonic logging while drilling a borehole traversing an earth formation

ABSTRACT

The present invention is directed to sonic logging while drilling. A transmitter and at least one receiver are mounted on a drill collar for performing sonic investigations of the formation traversed by a borehole. It has been discovered that a drill collar has a natural stop band or notch where acoustic energy propagating in this frequency range is severely attenuated. Thus, to reduce drill collar acoustic coupling, the transmitter is operated within this natural stop band of the drill collar. An imperforate stop band filter is also included between the transmitter and receiver to enhance the natural stop band. The transmitter is mounted transverse to the longitudinal axis of the drill collar, and is preferably mounted within means that reduce drill collar acoustic coupling. The transmitter preferably includes a material whose acoustic response is more favorable along its longitudinal axis relative to its radial axis, thereby directing the acoustic energy into the formation rather than the surrounding drill collar. Received acoustic energy waveforms are electronically processed to reliably detect, in the presence of drilling noise, those acoustic signals which are produced by the transmitter and transmitted through the formation.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation-in-Part of U.S. patent applicationSer. No. 07/839,969 filed 20 Feb. 1992, which was a Continuation-in-Partof U.S. Pat. application Ser. No. 07/548,169 filed on 5 Jul. 1990, nowabandoned, which was a Continuation-in-Part of U.S. Pat. applicationSer. No. 07/288,742 filed 22 Dec. 1988, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is directed to exploration and development ofsources of hydrocarbons and particularly to such exploration by acousticinvestigations of the properties of the formations traversed by aborehole. More specifically, the present invention is directed to anapparatus and method for transmitting an acoustic signal into theformation and for detecting the acoustic signal at a spaced detectorafter it has propagated through the formation. More particularly, thepresent invention is directed to methods of and apparatus for performingsonic logging while drilling.

2. Description of the Related Art

As known in the art, three operations occur during the process ofdrilling a borehole through an earth formation. These operations aretypically referred to in the art as tripping in, drilling and trippingout. Tripping in refers to lowering the drill bit in the existingportion of the borehole. Drilling refers to cutting the formation tolengthen, either horizontally or vertically, the existing portion of theborehole. Tripping out refers to pulling the drill bit back to thesurface, e.g., to check the bit, to change the bit, or to set casing.

As used herein, the term "logging while drilling" or "LWD" is defined asobtaining logging measurements with apparatus located on either a drillcollar or a drill string. These measurements may be obtained whiletripping in, tripping out, drilling, or any combination thereof.

After a well has been drilled, a number of wireline logs are ordinarilyobtained to reveal certain physical characteristics of the formation.Typically, resistivity, neutron porosity, and gamma ray densityinvestigations, the "Triple Combo", are made to acquire sufficientinformation to derive values of formation porosity and water saturation.Where additional information is desired, a sonic investigation issometimes performed in order to obtain another value of porosity, thespeed of propagation of sound in the rock and information on the bulkelastic properties of the formation. The information available from thesonic log is then useful in a variety of applications, including well towell correlation, determining porosity, determining the mechanical orelastic parameters of the rock and hence an indication of lithology,detecting overpressured formation zones, and enabling the conversion ofa seismic time trace to a depth trace from information acquired on thespeed of sound through the formation.

While wireline logs have in the past been the only way to obtain suchformation information in situ, recent developments in the oilfieldservice industry have effected alternatives to the Triple Combo wirelinemeasurements: namely the resistivity, the neutron porosity, and thegamma ray density measurements performed while drilling: the "LWD TripleCombo". Previously, the oilfield service industry has not developed a"while drilling" alternative to the fourth most commonly used log, thesonic log.

The basic principle of the sonic log is that an acoustic signal isgenerated at a transmitting transducer, the signal propagates throughthe formation to at least one receiving transducer, and the firstarrival is detected. From knowledge of the time of transmission and thetime of the first arrival of acoustic energy at the receiver afterhaving passed through the formation, the time of propagation of thesignal through the formation can be determined which is referred to asinterval transit time, Δt. Δt may then be used in the Wyllietime-average equation:

    Δt=Δtsolid(1-φ)+Δtfluid(φ)

to obtain porosity, φ. Δtsolid and Δtfluid are known from predeterminedknowledge of the speed of propagation of sound in various rocks andfluids and by knowing the types of rock and fluid in which theinvestigation is being made. As an alternative to the Wyllie timeaverage relationship, the more recent "Raymer, Hunt, Gardner"relationship may be utilized.

In order for a sonic tool to be able to detect the first formationarrival, the detected signal is preferably virtually free of energylaunched by the transmitter into the tool body which is then propagatedalong the tool body to the position of the receiver. Since the speed ofpropagation of sound in the tool body, which is normally steel, can bemuch higher than that of the formation rock, the tool propagated signalinvariably arrives before the formation arrival.

Techniques have been developed in the wireline logging industry forattenuating and/or slowing down the tool propagated signal so that theformation arrival may be detected without much interference from thetool propagated signal. These techniques have focused on creating a"tortuous path" in the sonde housing by incorporating perforations orslots through the sidewall of the tool's housing. See, e.g., thefollowing U.S. patents, incorporated herein by reference: U.S. Pat. No.3,191,141 to Schuster; U.S. Pat. No. 3,213,415 to Moser et al.; U.S.Pat. No. 3,271,733 to Cubberly Jr.; U.S. Pat. No. 3,364,463 to Pardue;U.S. Pat. No. 3,608,373 to Youmans; U.S. Pat. No. 4,020,452 to Trouilleret al.; and U.S. Pat. No. 4,850,450 to Hoyle et al.

The prior art has also proposed creating a tortuous path by placinginternal and external grooves into the sidewall of the sonde's housing.The internal and external grooves allegedly create an increased pathlength for the acoustic signal while retaining a constant housingthickness. See, e.g., the following U.S. patents, herein incorporated byreference: U.S. Pat. No. 3,190,388 to Moser et al.; U.S. Pat. No.3,191,141 to Schuster (FIGS. 21 and 23); U.S. Pat. No. 3,191,143 toPardue; and U.S. Pat. No. 3,493,921 to Johns (FIG. 3). Dissimilarmaterials can be placed in these grooves, as shown in U.S. Pat. No.3,190,388 to Moser et al.

Other tortuous paths extend the total length between transmitter andreceiver, thereby delaying the tool housing signal. See, e.g., thefollowing U.S. patents, incorporated herein by reference: U.S. Pat. No.3,191,142 to Lebourg; U.S. Pat. No. 3,213,415 to Moser et al.; U.S. Pat.No. 3,271,733 to Cubberly Jr.; U.S. Pat. No. 3,381,267 to Cubberly Jr.et al. and U.S. Pat. No. 3,493,921 to Johns (FIG. 4).

Other prior art acoustic isolating systems include mounting spacedtransmitters and receivers either on flexible material, such as rubber,as in U.S. Pat. No. 3,063,035 to Vogel et al., or on a material such asTeflon, whose acoustic velocity is slower than the housing material, asin U.S. Pat. No. 3,144,090 to Mazzagatti et al., both of which areincorporated herein by reference.

For several reasons, the expedient of providing openings or cuts thatextend through the side wall thickness of the member is clearlyunsatisfactory for a sonic investigation performed from a drill stringor drill collar. First, in the Logging While Drilling (LWD) environment,the investigating apparatus is incorporated into a drill collar or drillstring and therefore must be able to withstand the immense forces andaccelerations encountered thereon while drilling the borehole. If asignificant number of perforations were to be made through the sidewallof the drill collar, the collar would be weakened to the point that itwould no longer be able to withstand the forces imposed upon it by thedrilling process. Second, during drilling, mud is circulated through thedrill bit via the drill string including the collars, MWD tools, and LWDtools. The mud is necessary for several reasons, including clearing thedebris from the drill bit, as well as maintaining pressure on theformation to insure fluid isolation of independent zones. The pressuredifferential between the inside and outside of the drill collar istypically several thousand psi. Thus, if perforations or slots wereprovided in the drill collar, the drilling fluid would simply passthrough the slots and into the annulus and never reach the bit.

In theory, an ideal sonic transmitter for compressional wave loggingwith an LWD tool would excite an azimuthally symmetric (monopole) wavein the surrounding formation and the drill collar itself. However, ithas been found that due to various factors, such as the tremendousdrilling noise created by the drill bit and drill string interactingwith the formations, additional waves with other azimuthal behaviors aregenerated. These additional waves include dipole waves that vary inproportion to cos θ, and quadrupole waves that vary in proportion to cos2θ with θ being the azimuthal angle in a plane perpendicular to thelongitudinal axis of the drill collar. All three types of wavesgenerally contribute to the collar arrival and therefore must beattenuated.

From the above, it is clearly not practical to apply the wireline"tortuous path" technique of delaying tool arrivals to an LWD tool byproviding perforations or slots through an LWD drill collar. Yet, it isequally clear that an LWD sonic tool must be provided with some type ofmeans to sufficiently delay and/or attenuate acoustic signals travelingthrough the collar in order to effectively detect the formation signalwhile maintaining the physical integrity of the drill collar.

SUMMARY OF THE INVENTION

It has been discovered, through laboratory experiments and mathematicalmodeling of cylindrical drill collars, that due to the cylindricalgeometry of the drill collar, a natural notch or stop band exists formonopole wave propagation at a well defined, predetermined frequency atwhich the sound propagating in the drill collar is attenuated. This stopband occurs in the vicinity of the frequency of transition of thepropagation slowness of the first monopole collar mode from one slownessto another. At this frequency, acoustic energy couples well with thesurrounding medium and is subsequently lost to that medium.

It is proposed therefore to take advantage of this natural stop bandphenomenon to make a sonic type of measurement in the drillingenvironment. Accordingly, one object of the present invention is totransmit acoustic energy preferentially at or near the stop bandfrequency and to detect acoustic energy at or near the stop bandfrequency as a means of discriminating against the drill collarpropagated acoustic signal and of enhancing the detectability of theformation propagated acoustic signal.

In addition, it has been discovered that, having determined the naturalstop band frequency, a plurality of axially periodic, substantiallycircumferentially continuous sections of the drill collar (such asgrooves or ridges in the form of circumferential rings or helicalthreads), with acoustic propagation characteristics different from thedrill collar, may be provided in such a manner that the acoustic energyat the stop band frequency is further attenuated or filtered through acombination of reflection and destructive interference. A periodicallysectioned collar produces both slower and weaker collar arrivals, makingthe formation arrivals detectable in the stop band of the collar. Suchsections may either be circumferential mass loads on the drill collar orcircumferential grooves cut into the thickness of the sidewall of thedrill collar. These sections may be formed alternatively on the interioror the exterior of the drill collar and may be circular or helical.

It has further been discovered that a suitable arrangement of thesecircumferentially continuous grooves or ridges may be fashioned suchthat they also substantially filter or attenuate dipole and/orquadrupole collar waves in their respective stop bands. In addition, thecircumferential grooves or ridges may be arranged such that theirfiltering and/or attenuation properties for monopole, dipole, andquadrupole modes do not interfere with one another. In addition, thecircumferential grooves or ridges create substantially overlapping stopband attenuation notches for the three modes.

A further measure found to be beneficial in preventing the interferingpropagation of acoustic energy along the length of the drill collarincludes mounting both the acoustic transmitter and the acousticreceiver in acoustic isolation from the drill collar. Preferably, thetransmitter and receiver are oriented transversely to the longitudinalaxis of the drill collar so as to preferentially launch the acousticsignal toward the formation rather than along the length of the drillcollar.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a general illustration of a drilling rig and the drill stringwith a downhole acoustic logging tool of the present inventionincorporated into the drill string.

FIG. 2 is a schematic half cross-section of a periodically grooved drillcollar of the present invention with internal grooves.

FIG. 3 is a schematic half cross-section of a periodically grooved drillcollar of the present invention with external grooves and a transverselymounted transmitter and receiver.

FIG. 3a is a schematic partial cross-section of the outer wall of oneembodiment of a periodically grooved drill collar of the presentinvention having both internal and external grooves.

FIG. 3b is a schematic partial cross-section of the outer wall ofanother embodiment of a periodically grooved drill collar of the presentinvention having both internal and external grooves.

FIG. 4 is a plot of the acoustic propagation slowness for a tubulardrill collar versus frequency illustrating transition regions fromsmaller slowness to larger slownesses.

FIG. 5 is an illustration of a drill collar with randomly positioneddepressions extending into the material of the drill collar by afraction of the thickness of the drill collar.

FIG. 6 is a plot of amplitude versus frequency for a variety of samplesof drill collars including a smooth drill collar, a drill collar havingcircumferential grooves, and a drill collar having randomly positioneddepressions.

FIG. 6a is a plot of amplitude versus frequency of the monopole, dipole,and quadrupole modes for a smooth drill collar.

FIG. 6b is a plot of amplitude versus frequency of the monopole, dipole,and quadrupole modes for the drill collar illustrated in FIG. 3a.

FIG. 6c is a plot of amplitude versus frequency of the monopole, dipole,and quadrupole modes for the drill collar illustrated in FIG. 3b.

FIG. 7 shows a cross-sectional view of a drill collar through apreferred embodiment of the transducer assembly of the presentinvention.

FIG. 8 shows a top view of the outer steel plate shown in FIG. 10.

FIG. 9 shows a schematic cross-sectional view of a preferred embodimentof the receiver of the present invention.

FIG. 10 shows a cross-sectional view of a drill collar through apreferred embodiment of the acoustic receiver of the present invention.

FIGS. 11a and 11b are plots of typical received acoustic waveformsbefore processing by the acoustic signal processor of the presentinvention.

FIG. 12 is a plot of a typical acoustic waveform after processing by theacoustic signal processor of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 is a general illustration of a drilling rig and the drill stringwith a downhole acoustic logging tool of the present invention in theborehole. The rotary drilling rig shown comprises mast 1 rising aboveground 2 which is fitted with lifting gear 3 from which is suspendeddrill string 4 formed of drill pipes screwed one to another and havingat its lower end drill bit 5 for the purpose of drilling well 6. Liftinggear 3 consists of crown block 7, the axis of which is fixed to the topof mast 1, vertically traveling block 8, to which is attached hook 9,cable 10 passing round blocks 7 and 8 and forming, from crown block 7,on one hand dead line 10a anchored to fixed point 11 and on the otheractive line 10b which winds round the drum of winch 12.

Drill string 4 is suspended from hook 9 by means of swivel 13 linked byhose 14 to mud pump 15, which permits the injection of drilling mud intowell 6, via the hollow pipes of drill string 4. The drilling mud may bedrawn from mud pit 16 which may be fed with surplus mud from well 6. Thedrill string may be elevated by turning lifting gear 3 with winch 12 andthe drill pipes may be successively removed from (or added to) well 6and unscrewed in order to remove bit 5. These drill pipe raising andlowering operations require drill string 4 to be temporarily unhookedfrom lifting gear 3; the former is then supported by blocking it withwedges 17 in conical recess 18 in rotating table 19 mounted on platform20 through which the drill string 4 passes. The lowermost portion of thedrill string 4 may contain one or more tools, as shown at 30 forinvestigating downhole drilling conditions or for investigating theproperties of the geological formations penetrated by the bit 5 andborehole 6. Tool 30 shown is an acoustic logging tool of the presentinvention having at least one transmitter and a plurality of receiversspaced therefrom.

Variations in height h of traveling block 8 during drill string raisingoperations are measured by means of sensor 23 which may be an angle ofrotation sensor coupled to the faster pulley of crown block 7. Theweight applied to hook 9 of traveling block 8 may also be measured bymeans of strain gauge 24 inserted into dead line 10a of cable 10 tomeasure its tension. Sensor 23 and strain gauge 24 are connected bysignal lines 25 and 26 to a processor 27 which processes the measurementsignals and which incorporates a clock. Recorder 28 is connected toprocessor 27, which is preferably a computer.

Turning now to FIG. 2, the basic components of acoustic logging tool 30of the present invention are schematically illustrated. As in mostmeasurement while drilling operations, the tool is formed in alongitudinally extending body adapted for positioning in the borehole.In the drilling environment, this body is typically a heavy walled drillcollar 32 which is basically a thick walled cylinder (only a portion ofwhich has been shown) with a longitudinal central axis 51. Mounted onthe drill collar are acoustic transmitter 34 and acoustic receiver 36spaced therefrom. While only one receiver has been shown in FIGS. 2 and3, it is to be understood that it may be preferable to include an arrayof receivers (as shown in FIG. 1) and possibly more than one transmitterfor performing a borehole compensated sonic investigation. It shouldalso be understood that the Figures are not necessarily drawn to scale.For example, in practice, the distance between transmitter 34 andreceiver 36 and the number of grooves are much greater than shown.

It has been found that a member such as heavy walled drill collar 32supports a number of guided acoustic modes with slowness transitionregions. As an illustrative example, a plot of slowness versus frequencyfor the first monopole mode is shown in FIG. 4. For a typical smoothdrill collar having an 8.5 inch outside diameter and a 4 inch insidediameter, the first mode slowness changes from about 60 μs/ft below 9kHz to about 150 μs/ft above 12 kHz. The center transition frequency(point a, FIG. 4) occurs at about 10 kHz.

Second and third center transition frequencies (not shown) occur atabout 17 kHz and above about 22 kHz, respectively, corresponding toslowness transition regions in the second and third guided propagationacoustic modes of the drill collar. It is to be noted that theseslowness transition regions are dependent upon drill collar dimensions.

In this example, the first mode is highly attenuative at about 10 kHzbecause the collar vibrations couple very well to the fluid at thatfrequency. Thus, acoustic stop bands exist at the frequencies of thetransition regions at which acoustic coupling into the surroundingmedium is enhanced.

The center transition frequency is estimated by the present invention asthe frequency, f_(c), at which the wavelength of the extensional wave isequal to the mean circumference of the collar. That is:

    f.sub.c =kV/(d1+d2)

where

k=2n/π;

n=the ratio of the center transition frequency of the mode in questionto the center transition frequency of the first mode;

V=the bar extensional wave velocity of the collar,

d1=the inner diameter of the collar, and

d2=the outer diameter of the collar.

In the above example, n is equal to 1 for the center transitionfrequency of the first mode, n is equal to 1.7 for the center transitionfrequency of the second mode, and n is at least equal to about 2.2 forthe center transition frequency of the third mode. Depending on theproperties of the materials from which the drill collar is fabricatedand its size, the center transition frequency may lie in the range offrom about 3 to about 30 kHz.

Accordingly, advantage is taken of the stop band (or notch) by providingtransmitters and receivers which operate in the region of the stop bandfrequencies. In this manner, acoustic energy propagating in the drillcollar at the stop band frequency is attenuated, giving receiver 36 anopportunity to detect the energy that is launched into and propagatedalong the geological formation adjacent to tool 30.

While attenuation of the smooth collar propagated signal at the stopband frequency, in some circumstances, may be sufficient for thereceiving detector to acquire a reasonable formation signal, it is stillpossible to further enhance the effectiveness of the notch of the stopband by providing the drill collar with means intermediate thetransmitter and receiver for causing further acoustic attenuation at thepredetermined frequency of the stop band. Referring back to FIGS. 2 and3, this may be done by providing the drill collar 32 with axiallydiscontinuous, substantially circumferentially continuous portions 48having acoustic propagation characteristics different from those of thedrill collar material itself. As used herein, the term "axiallydiscontinuous, substantially circumferentially continuous portion" isintended to include not only a circumferential ring or band that windscontinuously around the drill collar, but also where the rings or bandsextend all the way around the drill collar except for a portion smallrelative to the drill collar circumference so long as the desirableacoustic effects of enhancing the stop band is obtained.

Portions 48 act primarily as reflectors that cause the acoustic energyto reflect back in the opposite direction. Preferably, such portionshave sides facing in the axial direction that are substantially normalto the longitudinal axis of the drill collar. Additionally, it has beendiscovered that a periodically spaced array of such portions, at theright spacing, may act as a means for not only reflecting and scatteringthe collar propagating acoustic energy but also for creating destructivewave interference which further reduces the amplitude of the acousticsignal within the stop band.

Such portions are illustrated in FIG. 2 as rectangular grooves 40separated by ridges or flats 42 that have preferably been formed on theinterior surface of drill collar 32. Formation of grooves 40 on theinterior of the drill collar 32 has a number of advantages compared toplacement of these grooves on the exterior of the collar. Theseadvantages include protection from abrasive contact with the boreholewall, a stronger and more rigid drill collar, as much as 50% reductionin stress concentrations, and the ability of the groove to have squarecorners as opposed to rounded corners.

The portions (spaces) 48 are preferably provided with the maximumattainable difference in acoustic propagation properties relative tothose of the material of the drill collar 32. Alternatively, thematerial of the portions 48 may be selected such that its speed ofpropagation of sound closely matches that of the drilling fluid so thata minimum of energy is reflected back into the drill collar 32 from theinterface between the material of the portion 48 and the drilling fluid.Spaces 48 may be filled with, e.g., epoxy, rubber, air, oil or othersuitable material. In the preferred embodiment, the material is air oroil , and is maintained by sleeve 50 which serves to trap the air or oiland to isolate it from the drilling fluid circulating on the inside ofcollar 32. In a preferred embodiment, sleeve 50 is isolated from collar32 by a gasket, e.g., rubber (not shown). Measures may be taken tohydraulically seal the ends of sleeve 50 with, for example, elastomerO-rings 49 which also serve to provide an acoustic impedance to theenergy launched by the transmitter 34.

Interior corners of grooves 40 are indicated at 46. Where grooves 40 areformed on the exterior of the drill collar 32 (as illustrated in FIG.3), interior corners 46 are preferably rounded in order to minimize thestress concentrations created by the formation of the grooves 40. Wheregrooves 40 are formed on the interior of the drill collar 32 (asillustrated in FIG. 2) where stress concentration is not as great aconcern, their interior corners are preferably as square as possible inorder to maximize the amount of acoustic signal reflected by thecollar/groove interface.

Turning now to FIG. 3, another embodiment is illustrated. As shown,grooves 40 have been formed on the exterior surface of drill collar 32and would be either empty (filled with the drilling mud) or filled withepoxy, fiberglass or some other material having significant resistanceto the abrasion expected through contact with the borehole wall duringthe drilling process. Grooves 40 are shown as having depths "r" andwidths in the longitudinal direction "L2". Grooves 40 are separated byridges or flats 42 having widths "L1". Generally, the widths L2, (andaxial spacings, L1 which may preferably be equal to L2) of the grooves40 may be determined in accordance with the following relationship:

    L1=L2=1/(4fS)

where

L1=the spacing between the grooves;

L2=the axial length of the groove;

f=the predetermined frequency; and

S=the slowness of an acoustic wave propagating in the body.

The thickness of the sidewall of drill collar 32 is indicated by "t" andis determined to satisfy both the attenuation requirement necessary tosuccessfully detect the formation arrival and a drill collar strengthrequirement. In one example, drill collar 32 may have an internaldiameter of 4 inches (10.2 cm), and an external diameter of 8.5 inches(21.6 cm) so that the thickness, t, of the drill collar is 2.25 inches(5.7 cm). In this situation, grooves 40 may have a depth r of 1.2 inches(3.05 cm) and widths L2 of 5 inches (12.7 cm) which is also the width L1of the ridges or flats 42 . The above listed dimensions have been foundto be satisfactory from both acoustic and mechanical perspectives. Thefurther the separation between transmitter 34 and receiver 36, thebetter the attenuation of the collar propagated signal. A separationdistance of 11 feet (3.35 m) has been found to be satisfactory in mostconditions to enable the detection of a reasonable formation arrival.

While it has been found that the collars provided with stop band filtersillustrated in FIGS. 2 and 3 are effective in attenuating monopole formsof acoustic energy traveling within the collar, the filter is not aseffective in attenuating other modes, such as dipole and quadrupolegenerated, for example, by drilling noise and nonperfectly symmetrictransmitters. Accordingly, FIG. 3a illustrates another preferredembodiment of the present invention for creating a more effectiveacoustic stop band filter within a drill collar. In FIG. 3a, grooves 72and grooves 74 are formed on both the inside and outside surfaces 76 and78, respectively, of drill collar 70. Grooves 72 and 74 are preferablystaggered, i.e. radially misaligned, so as to maintain a minimum wallthickness and thereby provide collar 70 with sufficient strength andrigidity to survive the drilling process. It has been found thatmonopole and quadrupole acoustic waves are sufficiently attenuated when,for an 8.25 inch (21.0 cm) O.D. and 4.0 inch (10.2 cm) I.D. drillcollar, the depth r₁ and length L₁ of inner grooves 72 are 1.2 inches(3.0 cm) and 1.6 inches (4.1 cm), respectively, while the depth r₂ andlength L₂ of outer grooves 74 are 0.75 inches (1.2 cm) and 1.6 inches(4.1 cm), respectively, with L₃ and L₄ of flat portions 77 and 79,respectively, both being 4.8 inches (12.2 cm), resulting in a period forboth inner and outer groove patterns of 6.4 inches (16.3 cm). It hasbeen found that the rapid increase in monopole mode slowness withfrequency above the transition frequency requires this short period foreffective attenuation in this frequency region.

FIG. 3b illustrates yet another preferred embodiment of the presentinvention for creating an effective acoustic stop band filter within adrill collar. In FIG. 3b, a plurality of small inner grooves 82, largeinner grooves 83, and outer grooves 84 are formed in both the inside andoutside surfaces 86 and 88, respectively, of drill collar 80. Innergrooves 82 and 83 and outer grooves 84 are also preferably radiallymisaligned to maintain a minimum wall thickness. In this embodiment, ithas been found that monopole, dipole, and quadrupole acoustic waves areeffectively attenuated when, for an 8.25 inch (21.0 cm) O.D. and 4.0inch (10.2 cm) I.D. collar, the depth r₅ and length L₅ of small innergrooves 82 are 1.2 inches (3.0 cm) and 1.0 inches (2.5 cm),respectively; the depth r₆ and length L₆ of large inner grooves 83 are1.2 inches (3.0 cm) and 2.0 inches (5.1 cm), respectively; and depth r₇and length L₇ of outer grooves 84 are 0.6 inches (1.5 cm) and 1.0 inches(2.5 cm), respectively, with flat portion 85 between inner grooves 82and 83 having a length L₈ of 3.0 inches (7.6 cm), thereby yielding amixture of 4 inch (10.2 cm) and 5 inch (12.7 cm) periods. Outer grooves84 are spaced such that they preferably fall in the middle of flatportions 85, thereby also yielding a mixture of 4 inch (10.2 cm) and 5inch (12.7 cm) periods.

FIG. 5 illustrates another embodiment of the present invention forforming an acoustic stop band on a drill collar. In this embodiment, themeans for producing a stop band are not circumferentially continuousportions but rather are depressions 110 formed in the surface (exterioror interior) of the drill collar at random locations. In the specificexample illustrated, which has been tested experimentally with a onetenth scale model, depressions 110 comprise holes (filled or unfilled asdescribed above with regards to grooves 40) with diameters and depths ofone tenth of an inch (0.25 cm). The side wall of the one tenth scalemodel is comprised of material 112 with a thickness of 0.15 inches (0.38cm). Unlike the case of a wireline sonde having holes which areuniformly spaced, are formed entirely through the sonde housing, andwhich attenuate acoustic energy all along the spectrum, depressions 110enhance the naturally occurring stop band (within a limited frequencyrange) of the drill collar.

FIG. 6 is a representation of the frequency spectra of experimental dataderived from three one tenth scale models using a ring source and a ringreceiver. The one tenth scale drill collars used in the experiment hadinternal diameters of 0.5 inches (1.3 cm) and outside diameters of 0.8inches (2.0 cm). In FIG. 6, curve A represents the spectrum of a smoothdrill collar, curve B represents the spectrum of a drill collar havingcircumferential exterior grooves through 40% of the thickness of thedrill collar, and curve C represents the spectrum of a drill collarhaving random circular depressions through 67% of the thickness of thedrill collar.

As can be seen, the attenuation spectrum of curve A shows a natural stopband at approximately 100 kHz (which is equivalent to about 10 kHz atfull scale ) with a band width of approximately 20 kHz (2 kHz fullscale). As can be seen from curve B, the addition of external groovesnot only deepens the stop band but widens it. Curve C, the curve from aone tenth scale drill collar having randomly positioned circulardepressions, has a stop band that has been broadened with some deepeningrelative to the smooth collar stop band of curve A. It is expected thata drill collar having depressions with a range of diameters wouldexhibit further broadening of the stop band of curve C. Othercombinations of internal and/or external grooves and/or randomdepressions of the same or different sizes can be utilized to produceother desirable variations. As will be realized by those skilled in theart, dimensions and frequencies for full size drill collars having thefeatures of the models of FIG. 6 may be obtained by applying the scalefactor of 10.

FIG. 6a is a representation of the frequency spectra of the waveform ofa smooth 8.5 inch×4.0 inch (21.6 cm×10.2 cm) drill collar. As can beseen in FIG. 6a, the collar exhibits a relatively deep monopole naturalstop band or notch centered at about 10 KHz while both the dipole andquadrupole signals show very little attenuation at this frequency.

FIG. 6b is a representation of the frequency spectra of the waveformfrom a collar provided with the stop band filter of the presentinvention shown in FIG. 3a. The monopole attenuation notch is centeredon 10 kHz and is about 3.8 kHz wide at the half-minimum points. Ascompared to the waveform of FIG. 6a, the dipole curve in this frequencyonly shows a slight improvement in attenuation. However, the quadrupoleattenuation is substantial.

FIG. 6c is a representation of the frequency spectra of the waveformfrom a collar provided with the stop band filter of the presentinvention shown in FIG. 3b. In FIG. 6c, the dipole notch is about asdeep as the monopole notch and the frequency band of overlap is about3.5 kHz centered at about 11.5 kHz, thus providing an excellent overlapof the two attenuation notches. Depending on the relative amplitudes ofmonopole and dipole collar excitation with a given transmitter design,the center frequency and band width of the pulse can be adjusted tominimize the most troublesome component; slightly lower frequenciesfavor monopole attenuation, while slightly higher frequencies favordipole attenuation. Quadrupole attenuation again remains excellent.

Receiver sensitivity to dipole arrivals can be further reduced bysumming pairs of matched receivers located at diametrically opposedpositions and with the axis between them aligned with that of thetransmitter stack. Similarly, dipole and quadrupole arrivals can besimultaneously reduced by summing four receivers arranged at 90°intervals.

Returning to FIG. 3, there is shown one possible design of transmitter34 and receiver 36 and their mounting arrangement. It is a desirableexpedient to minimize the acoustic coupling between the transmitter 34and the drill collar 32 so that the signal propagated by the collar 32starts as small as possible. Thus, the transmitter 34 is preferablymounted transverse to the longitudinal axis 51 of the drill collar 32.Ideally, then, a minimum of acoustic energy is launched into the drillcollar 32 and a maximum amount of the energy is launched into thegeological formation.

As can be seen from FIG. 3, transducers 56 and 58 are generallycylindrical in shape. Preferably, transducer 56, acting as thetransmitter, functions in the range of 3-30 kHz. Transducers maycomprise a series of stacked disks of piezoelectric material which areelectrically driven to vibrate or resonate in unison. The stack of disksmay include mass loads (not shown) at one or both ends in order to tunetheir resonant frequencies to the stop band frequency of the drillcollar 32. In the preferred embodiment, transmitter 34 is a narrow bandtransducer emitting a majority of its signal at the frequency of thestop band of the drill collar 32 while receiver 36 is a broader bandtransducer spanning the stop band of the drill collar 32. The broaderband of the receiver 36 enables a maximum amount of acoustic energy tobe received from the adjacent formation after having traveled throughthe formation from transmitter 34.

Transducer 56 may be mounted in the drill collar 32 in transverselyextending tube 52. Tube 52 serves to physically isolate and seal thetransducer 56 from the drilling fluid on the interior of tool 30.Transducer 56 (and 58) may be acoustically isolated from its mountingtube 52 by means of resilient O-rings 60 which produce an air (or fluidfilled) gap 57 between the transducer 56 and the sides of tube 52.Additional acoustic isolation may be achieved between the transducer 56and the drill collar 32 by resiliently mounting the tube 52 in thecollar (not shown).

Transmitter 34 has been illustrated as comprising a single transducerextending the width of the drill collar while the receiver 36 has beenillustrated as comprising a pair of transducers 58 at opposite sides ofthe drill collar. Is should be apparent, however, that both thetransmitter 34 and receiver 36 may be of either design. Where thetransmitter 34 (or receiver 36) comprises a pair of transducers, it maybe operated in either a monopole (point source) or a dipole (pointforce) mode.

While drill collars having internal and external portions formed in themfor producing acoustic attenuation in a stop band have been described,other means may be found to achieve similar results. For example, ratherthan machining grooves in the material of the drill collar, it might befound preferable to provide portions which comprise mass loads by fixingbands of material on the exterior or interior of the drill collar. Thebands may be affixed to the drill collar by means of heat shrinking orby hot winding a helical strip to the exterior of the collar.

As stated above with reference to FIG. 4, a drill collar acts as awaveguide which conducts several acoustic modes. For each mode, soundtravels therethrough at all times, with lower frequencies arrivingrelatively early and higher frequencies arriving relatively later.However, it has been discovered that a transition frequency range existswhereat acoustic energy propagating in this transition frequency rangeis radiated into the surrounding wellbore fluid, e.g., mud. It has alsobeen discovered that the range of the transition frequency is dependentupon the dimensions of the drill collar. The center frequency of thisrange is estimated to be as follows: ##EQU1## where k=2n/π;

n=the ratio of the center transition frequency of the mode in questionto the center transition frequency of the first mode;

V=the bar extensional wave velocity of the drill collar, defined as thesquare root of Young's Modulus over the density of the drill collar(steel);

d₁ =the inner diameter of the drill collar; and

d₂ =the outer diameter of the drill collar.

Thus, by locating an acoustic source and receivers in a drill collar andoperating them in a relatively narrow band of frequencies defined by thedrill collar's transition frequency range, the natural stop bandproperties of the drill collar can be advantageously employed.

As explained previously, the prior art teaches perforating, i.e.,puncturing, a sonde housing in order to attenuate tool arrivals.Although this technique is adequate for conventional wirelineapplications, perforating a drill collar is not feasible in the loggingwhile drilling environment. For example, perforating a drill collarwould render the drill collar structurally inadequate to withstand theforces imposed thereupon by the drilling process, and also destroy thecollar's ability to transmit high pressure drilling fluid to the drillbit.

The inventors of the present invention have noted that a drill collarcomprising grooves or other depressions therein, yet which do not fullyperforate the drill collar, produces a stop band filter which attenuatessignals traveling therethrough in a predetermined frequency band. Moreimportantly , such grooves or depressions do not adversely affect thestructural integrity of the drill collar.

As used herein, the term "imperforate stop band filter" is defmed as aplurality of grooves or other depressions which do not perforate thedrill collar, and which cause the energy transmitted through the drillcollar between the transmitter and receiver to be substantiallyattenuated within a predetermined frequency band. The prefix"imperforate" is chosen to emphasis the fact that the grooves or otherdepressions do not destroy the fluid isolation established by anon-perforated drill collar.

As known in the art, the wavelength, λ, is defmed as velocity overfrequency. Written in terms of slowness, where slowness is defmed as theinverse of velocity:

    λ=1/(fS)

where

f=frequency; and

S=slowness of the drill collar sound wave.

The imperforate stop band filter preferably acts as a half-wavelengthattenuator. Thus, twice the period of the filter equals the wavelength.Defining the period as the spacing between adjacent grooves or otherdepressions, L1, and the width of each groove or other depressions, L2:

    λ/2=L1+L2

Combining the above two previous equations:

    L1+L2=1/(2fS)

In the preferred embodiment, L1 equals L2. Thus the equation:

    L1=L2=1/(4fS)

In the present invention, the imperforate stop band filter preferablycomprises periodically spaced grooves placed on the interior wall of thedrill collar, and are preferably located the full 360 degrees around thedrill collar. Other embodiments can also be chosen, for example,non-periodic grooves, grooves which do not fully traverse thecircumference of the drill collar, a helical shaped groove, or randomdepressions. It is preferable to locate the grooves or other depressionson the interior surface of the drill collar. Alternatively, the groovesor other depressions can be located on the exterior surface thereof,optionally including a sleeve placed about the drill collar, at leastover the grooves or other depressions, e.g., to increase the structuralintegrity of the drill collar and/or to allow the grooves to be filledwith a dissimilar fluid such as air.

An imperforate stop band filter can be designed for virtually anyfrequency band. In the present invention, the inventors have designedthe imperforate stop band filter about the frequency range of thenatural stop band of the drill collar. In this way, the natural stopband of the drill collar is improved.

For example, in a smooth collar having an 8 inch (20.3 cm) OD and a 4inch (10.2 cm) ID, a natural stop band exists at about 9 to 11 kHz.Providing 0.6 inch (1.5 cm) deep external grooves extends the stop bandto about 8.5 to 13 kHz. Providing 1.0 inch (2.5 cm) deep grooves extendsthe stop band even farther, to about 6 to 13 kHz. Given a drill collarof 8.4 inch (21.3 cm) OD, 4 inch (10.2 cm) ID, the smooth drill collar'snatural stop band of about 9.5 to 11.5 kHz was extended to about 6.5 to14 kHz with the addition of 1.2 inch (3.0 cm) deep internal grooves. Inthese examples, L1 equals L2 equals 5 inches (12.7 cm).

Turning now to FIG. 7, a cross-sectional view of the drill collarthrough a preferred embodiment of the acoustic transducer of the presentinvention is illustrated. Drill collar 701 houses transversely mountedtransmitter 703, steel housing 704, fluid channel 705, pad 707 locatedbetween transmitter 703 and housing 704, securing clips 709 and 711,placement clips 713 and 715, inner sleeve 717, and protective coverplates 719 and 721 to protect the transducer assembly from the externalenvironment.

Transmitter 703 can be any material, but is preferably a material whoseacoustic response is more favorable along its longitudinal axis relativeto its radial axis. In this way, the acoustic response is directed moretowards the formation than the surrounding drill collar. Thus, thetransmitter 703 is preferably lead meta-niabate and more preferably leadtitanate. As will be appreciated by those skilled in the art, othermaterials having a relatively high ratio of d33 to d31 can also bepreferably employed.

The transmitter 703 preferably comprises a plurality of discs withelectrodes on the faces and poled to resonate in a thickness mode. Metalelectrodes are preferably bonded between the discs, and the discs arepreferably wired in parallel. Eccobond 276, along with catalyst 17-M1,both available from Emerson-Cuming, is preferably used to bond theceramic discs and electrodes into a stack. The adhesive layer betweenthe metal electrode and the ceramic is typically 1 mil thick. The discsare preferably arranged so that sides of like polarity face each other.

The metal electrodes are preferably 2 mil nickel, although othermaterials, e.g., copper, and/or other thicknesses can be employed. It ispreferable to clean the electrodes before bonding, e.g., with #280wet-dry paper, 3M scotch-brite and acetone.

It is preferable to keep a constant pressure on the stack while theadhesive is curing. The curing sequence is preferably four hours at 75°C., 8 hours at 121° C. and 3 hours at 175° C.

As will be appreciated by those skilled in the art, operating atransmitter in a resonance mode produces maximum acoustic output. Inorder to minimize the overall length of the transmitter, as discussed inmore detail below, the transmitter is preferably designed as a halfwavelength resonator resonant in its fundamental mode.

As known in the art, the wavelength, λ, is defmed as velocity overfrequency.

    λ=V.sub.c /f.sub.f

where

V_(c) =the velocity of sound through the material of the transmitter;and

f_(r) =the resonant frequency.

As stated above, the transmitter is preferably designed as a halfwavelength resonator resonant in its fundamental mode. Thus:

    d=λ/2

where

d=the total length of the transmitter.

Combining the above two equations, and solving for total length of thetransmitter:

    d=V.sub.c /2f.sub.f

Thus, as velocity of sound through the transmitter is a fixed quantity,total stack length is determined by the resonant frequency desired.

In situations where the dimensions of a drill collar are not adequate tocontain the transmitter length, the resonant frequency can be modifiedso that the transmitter fits in the drill collar. Alternatively, asection of the transmitter can be replaced with a material whoseacoustic length matches the acoustic length of the transmitter sectionremoved but whose physical length is smaller.

The total length of the transmitter, d, in the above equation refersboth to the physical as well as the acoustic length of the transmitterwhere the transmitter material is homogeneous. Otherwise, d representsthe total acoustic length of the transmitter. Sound travels through somematerials slower than through others. Thus, a shorter physical length ofthe former is required to produce the same acoustic length of thelatter. The resulting substitution therefore produces a transmitterhaving an identical acoustic length but a shorter physical length. Asthe total acoustic length remains the same, so does the resonantfrequency. The substituted piece can be placed at either terminal end orlocated somewhere therebetween. Any material whose acoustic velocity isless than the acoustic velocity of the original material will suffice.

Commonly employed drill collars range from 6.5 inches (16.5 cm) OD, 4.5inches (11.4 cm) ID, to 9.5 inches (24.1 cm) OD, 5.7 inches (14.5 cm)ID. Thus, the center frequency will have a range of about 8.5 kHz toabout 12 kHz. In the preferred embodiment for use with a drill collar of8.5 inches (21.6 cm) OD, 4 inches (10.2 cm) ID, the transmitter willpreferably comprise about 34 discs of lead titanate, each disc being 0.2inches (0.5 cm) thick and 0.7 inches (1.8 cm) in diameter.Alternatively, the transmitter will comprise two sections of leadtitanate and a center section of lead zirconate titanate. Each leadtitanate section will comprise about 10 discs, the center section oflead zirconate titanate will comprise about 8 discs, each disc being 0.2inches (0.5 cm) thick and 0.7 inches (1.8 cm) in diameter. In bothcases, the transmitter will have a center frequency of about 10 kHz anda bandwidth between about 8 kHz and about 12 kHz.

Fluid channel 705 further isolates acoustic energy transmission from thesource to drill collar 701. The channel can be filled with any materialor composition, e.g., solid, liquid and/or gas. In the preferredembodiment, fluid channel 705 is filled with air. The fluid channel ispreferably about 20 to 30 thousandths of an inch (0.50-0.76 mm) thick,and is preferably formed by the intersection of housing 704 and innersleeve 717.

Acoustic transmitter 703 is further isolated from both drill collar 701and housing 704 by pad 707. Pad 707 further acts to secure transmitter703 in place, as does securing clips 709 and 711, placement clips 713and 715, and protective cover plates 719 and 721. Cover plates 719 and721 further act to protect the transmitter assembly from the hostileexternal environment. In the preferred embodiment, pad 707 is rubber,securing clips 709 and 711 are plastic, e.g., PEEK, placement clips 713and 715 are steel, inner sleeve 717 is steel, and cover plates 719 and721 are steel.

As will be appreciated by those skilled in the art, drilling fluid,e.g., mud, flows through the interior of the drill collar. It istherefore preferable that inner sleeve 717 at least be tapered about theexterior of the transmitter assembly to allow the drilling fluid to flowtherearound. More preferably, inner sleeve 717 is extended throughoutthe interior of the drill collar, allowing, e.g., wires for electricalconnections to the transmitter and receivers and the like to be placedwithin the drill collar, providing protection for the receiver andassociated electronics (FIG. 10), and a fluid seal between internalgrooves for placement of a fluid therein (FIG. 2). In the preferredembodiment, inner sleeve 717 is isolated from drill collar 701 via agasket, e.g., rubber (not shown). Additionally, inner sleeve 717 canalso include an imperforate stop band filter thereon to reduce anyacoustic coupling.

Turning now to FIG. 10, a cross-sectional view of the drill collarthrough a preferred embodiment of the acoustic receiver assembly of thepresent invention is illustrated. Each receiver 900, explained in moredetail with reference to FIG. 9, comprises a stack of ceramic discs 901and electrodes 903, positive lead 905 and ground lead 907. In thepreferred embodiment, each disc is 0.1 inches (0.25 cm) thick, 0.5inches (1.27 cm) in diameter. Preferably, the receiver stack comprisessimilar materials and is assembled in similar fashion to the sourcestack as described in detail above, i.e., lead titanate discs, nickelelectrodes with adhesive between the discs and electrodes.

Each receiver is preferably placed in drill collar 1001 as shown withreference to FIG. 10, wherein the receiver stack is protected by rubbersleeve 1003, steel sleeve 1005, steel tube 1007, inner plastic plate1009 and outer steel plate 1011. Pins 1013 and 1015 help secure plate1009 in place, while bolts (not shown) secure plate 1011 to drill collar1001.

Leads 905 and 907 from the stack are accessed via feedthrough 1027 toreceiver electronics 1040 (shown schematically). Receiver electronics1040 include one or more acoustic signal processors (ASP's) 1041.Acoustic signal processors 1041 process the acoustic waveforms receivedfrom the formation to extract those waveforms which are produced by thetransmitter, as will be discussed in detail in conjunction with FIGS.11a, 11b and 12. O-rings 1017, 1019, 1023 and 1025 provide fluidisolation for receiver electronics 1040, located beyond feedthrough1027, from possible mud infiltration via plate 1011, plate 1009 and/orsteel housing 1021. In the preferred embodiment, the receiver stack issurrounded by rubber 1029 to isolate the receiver stack from drillcollar 1001. As described above, the receiver electronics 1040 andreceiver leads 905 and 907 are preferably protected by inner sleeve 717.

In order for acoustic energy emitted from the transmitter to be receivedat receiver 900, outer steel plate 1011 preferably includes cut-out1031. A top view of plate 1011 is shown with reference to FIG. 8,wherein holes 801 and 803 are shown for mounting the plate to the drillcollar 1001, as described above.

Optionally, a fluid chamber can be placed about the receiver assembly.Fluid channel 1033 further attenuates acoustic energy transmissionthrough the drill collar 1001 to receiver 900. The channel 1033 can befilled with any material or composition, e.g., solid, liquid and/or gas.In the preferred embodiment, fluid channel 1033 is filled with air. Thefluid channel is preferably formed by the intersection of housing 1021with drill collar 1001 and inner sleeve 717.

Referring now to FIG. 11a, there is illustrated a typical acousticwaveform W1, as is received by the receivers of the present inventionwhile drilling is proceeding. Waveform W1 is a composite of the randomnoise generated by the drilling process, the acoustic signal produced bythe transmitter and transmitted through the attenuating portion of thedrill collar, and the acoustic signal produced by the transmitter andtransmitted through the formation. The Full Scale amplitude limits, bothpositive (+) and negative (-), at which the receiver electronicis 1040saturate are illustrated in FIG. 11a. Time T0 corresponds to the firstfiring of the transmitter. It should be noted that, in the time intervalT2-T1, the amplitude of waveform W1 does not exceed the full scalelimits of the receiver electronics 1040.

FIG. 11b illustrates a typical acoustic waveform W2 such as may bereceived by the receivers during a time interval corresponding to thatin FIG. 11a, but beginning with the subsequent firing of thetransmitter. The amplitude of waveform W2, in contrast to waveform W1,exceeds the full scale limits of the receiver electronics 1040 forportions of the time interval T2-T1. Similar waveforms W3 through Wn areacquired by the receivers and stored in the memory of ASP 1041 for eachtime interval beginning with each subsequent firing of the transmitter.This process continues until the transmitter firing sequence, comprisinga large number of individual firing events, is completed.

The processing of the stored waveforms, by which the acoustic signalsproduced by the transmitter and transmitted through the formation areextracted from the waveforms W1 through Wn, is illustrated in FIGS. 11a,11b and 12. As shown in FIGS. 11a and 11b, for a time interval Dsubsequent to each firing of the transmitter, the amplitude of eachreceived waveform is not evaluated by the ASP 1041. In the preferredembodiment, no processing occurs during the time interval D as a meansto eliminate transmitter to receiver crosstalk. Waveform processingbegins at time T1. It will be understood, however, that processing maybegin as early as time T0. The time interval D is fully programmable.

Following the time interval D, the amplitudes of each waveform duringthe time interval designated as the Noise Window are evaluated. Theduration of the Noise Window is also fully programmable. Any waveformwhich has a positive or negative amplitude exceeding the full scalevalue is rejected by ASP 1041 and is not considered in the subsequentportion of the processing sequence. The exception being that, when allprevious waveforms acquired during a given transmitter firing sequencehave been rejected, the final waveform Wn will be retained for furtherprocessing whether or not it has amplitudes exceeding full scale withinthe Noise Window. While in the preferred embodiment the Full Scaleamplitude limits are selected as the maximum amplitudes which can beacquired by receiver electronics 1040, it will be understood that theFull Scale limits could be set at other amplitudes. In FIGS. 11a and11b, waveform W1 represents a typical waveform which would be retainedfor further processing. Waveform W2 represents a typical waveform whichwould be rejected due to its extreme amplitudes.

Once each waveform W1 through Wn has been evaluated over the NoiseWindow, those waveforms which have been retained for further processingare summed or stacked within ASP 1041. Because the amplitudes andfrequencies of the drilling noise component of each composite waveformW1 through Wn are random, when sufficient waveforms with amplitudeswithin the Full Scale range are stacked the random noise components tendto cancel each other, leaving as a resultant waveform the acousticsignal produced by the transmitter and transmitted through the earthformation to the receivers.

A typical resultant or stacked waveform is illustrated in FIG. 12. Thisstacked waveform Wf represents the formation arrival signal for theinterval of the formation which has been drilled during the transmitterfiring sequence. The waveform Wf may be evaluated, following furtherprocessing, to determine the speed of acoustic transmission within theformation.

Before detailing the further processing of stacked waveform Wf,selection of the parameters which influence the processing resulting inwaveform Wf will be briefly described. As downhole electronic processingrequires power which is supplied by a battery pack in the acousticlogging tool, and because minimum data acquisition rates must bemaintained to obtain an accurate log of the formation, it is imperativethat the amount of processing be minimized and limited to only thatwhich is necessary to obtain a stacked waveform Wf of suitable quality.Hence, the number of acquired waveforms Wn should be only that numbernecessary to sufficiently eliminate random drilling noise when thosewaveforms with amplitudes within the Full Scale range are stacked. Thenumber of waveforms which must be stacked of course increases when highdrilling noise conditions are encountered. The ASP 1041 of the preferredembodiment is capable of stacking up to 255 waveforms, with the numberof waveforms to be stacked being preselected, based upon anticipateddrilling conditions, before the tool is placed downhole. It will beunderstood by those skilled in the art that an ASP capable of stackingmore than 255 waveforms could be provided and that the number ofwaveforms to be stacked may be changed in response to a signal from thesurface after the tool is placed downhole.

Referring again to FIG. 12, the illustrated stacked waveform Wfrepresenting the formation arrival signal comprises an initial formationcompressional arrival portion Wc, a subsequent formation shear arrivalportion Ws, and a final tube wave arrival portion Wt. The tube wavearrival portion Wt represents acoustic energy transmitted to thereceivers through the mud in the borehole annulus. Further processing ofthe waveform Wf proceeds by evaluating the amplitudes of the waveform Wfover the time interval designated as the Gain Window. The duration ofthe Gain Window is fully programmable and is set to include all or partof the formation shear arrival portion Ws of waveform Wf while excludingthe higher amplitude tube wave arrival portion Wt. As will be noted inFIG. 12, the duration of the Gain Window is generally longer than thatof the Noise Window, which is normally programmed to exclude thoseportions of waveforms W1 through Wn which include formation sheararrivals and tube wave arrivals. The Noise Window and the Gain Windowcould, of course, be programmed to be of equal length.

The portion of the waveform Wf subsequent to the Gain Window generallyrepresents the tube wave arrival portion Wt and is discarded by ASP1041. The amplitudes of the remaining portion of waveform Wf, which iswithin the Gain Window, are then evaluated against the Gain-Up and theGain-Down amplitudes which are also fully programmable. Again, while theGain-Down amplitude is generally greater than the Gain-Up amplitude,these may be programmed at the same amplitude. If all waveformamplitudes are less than the Gain-Up amplitude, the gain, which isapplied to waveforms W1 through Wn received during the next transmitterfiring sequence, is automatically increased by ASP 1041. If any waveformamplitude is greater than the Gain-Down amplitude, the gain isautomatically reduced. Following gain processing, the portion of thewaveform Wf within the Gain Window is evaluated to determine thepropagation speeds of the compressional and shear waves in the intervalof the formation drilled during the transmitter firing sequence. Thisprocess of transmitter firing sequences and waveform stacking iscontinuously repeated during the drilling process to provide acontinuous log of the acoustic properties of the formation.

Although illustrative embodiments of the present invention have beendescribed in detail with reference to the accompanying drawings, it isto be understood that the invention is not limited to those preciseembodiments. Various changes or modifications may be effected therein byone skilled in the art without departing from the scope or spirit of theinvention.

For example, although the present invention has been described withreference to a drill collar, the present invention is also applicable todrill strings as well as to logging sondes operable on conventionalwirelines. As appreciated by those in the art, sonic wireline loggingsondes often have acoustic energy propagating through the sonde housing,commonly referred to as tool arrivals. Several prior art techniquesexist for removing the tool arrivals. It is contemplated that aspects ofthe present invention are applicable for reducing and/or eliminatingwireline tool arrivals.

For example, a transversely mounted transmitter, isolating thetransmitter from the sonde housing via a fluid channel, operating thetransmitter at or near the natural stop band frequency of the sondehousing, providing a imperforate stop band filter between thetransmitter and receivers, operating the transmitter at or near thecenter frequency of the imperforate stop band filter, and/or setting thecenter frequency of the imperforate stop band filter at or near thecenter frequency of the sonde's natural stop band filter.

We claim:
 1. A method for performing sonic logging while drilling aborehole traversing an earth formation, including drilling the boreholewith a drill string having a drill bit at its lower end and drillingfluid in the borehole surrounding the drill string, the steps of saidmethod comprising:a) drilling with a drill collar incorporated into thedrill string; b) transmitting, from a location on said drill collar,acoustic energy into the surrounding earth formations; c) receiving, ata location on said drill collar, acoustic energy returned from thesurrounding earth formations; d) providing at least one output relatedto the received acoustic energy, said at least one output comprising aplurality of waveforms; and e) processing said at least one output todetermine at least one characteristic of said earth formations, saidprocessing comprising summing said plurality of waveforms to obtain aresultant stacked waveform.
 2. A method for performing sonic loggingwhile drilling a borehole traversing an earth formation, includingdrilling the borehole with a drill string having a drill bit at itslower end and drilling fluid in the borehole surrounding the drillstring, the steps of said method comprising:a) drilling with a drillcollar incorporated into the drill string; b) transmitting, from alocation on said drill collar, acoustic energy into the surroundingearth formations; c) receiving, at a location on said drill collar,acoustic energy returned from the surrounding earth formations; d)providing at least one output related to the received acoustic energy,said at least one output comprising a first plurality of waveforms; ande) processing said at least one output to determine at least onecharacteristic of said earth formations, said processing comprisingestablishing a first maximum amplitude, comparing the amplitudes of eachof said first plurality of waveforms to said first maximum amplitude,summing those waveforms having amplitudes less than said first maximumamplitude to obtain a resultant stacked waveform.
 3. The method of claim2, wherein the amplitudes of each of said first plurality of waveformsare compared to said first maximum amplitude over only a portion of eachwaveform.
 4. The method of claim 3, wherein said resultant stackedwaveform comprises an initial formation compressional arrival portion, asubsequent formation shear arrival portion, and a final tube wavearrival portion; andwherein said portion of each of said first pluralityof waveforms, over which the amplitudes are compared to said firstmaximum amplitude, excludes the portion of each of said first pluralityof waveforms corresponding to said tube wave arrival portion of saidresultant stacked waveform.
 5. The method of claim 4, wherein said atleast one output further comprises a second plurality of waveforms andwherein said processing step e) further comprises:establishing a minimumamplitude and a second maximum amplitude; comparing the amplitudes ofsaid stacked waveform to said minimum amplitude and to said secondmaximum amplitude over a portion of said stacked waveform; applying again to each of said second plurality of waveforms to increase itsamplitude if all amplitudes in said comparison portion of said stackedwaveform are less than said minimum amplitude; and applying a gain toeach of said second plurality of waveforms to decrease its amplitude ifany amplitude in said comparison portion of said stacked waveform isgreater than said second maximum amplitude.
 6. The method of claim 5,wherein said comparison portion of said stacked waveform substantiallyexcludes said tube wave arrival portion of said stacked waveform.
 7. Themethod of claim 6, wherein said processing step e) furthercomprises:determining from said comparison portion of said stackedwaveform the propagation speed of acoustic energy in said earthformation.
 8. The method of claim 7, wherein said acoustic energycomprises compressional or shear waves.
 9. A method for performing soniclogging while drilling a borehole traversing an earth formation todetermine characteristics of said earth formation, the steps of saidmethod comprising:a) drilling a borehole with a drill collar supportinga drill bit; b) transmitting acoustic energy about a given centerfrequency with a transmitter mounted on the drill collar; c) receivingacoustic energy with at least one receiver mounted on the drill collar;d) providing at least one output related to the received acousticenergy, said at least one output comprising a plurality of waveforms;and e) processing said at least one output to determine at least onecharacteristic of said earth formation, said processing comprisingestablishing a first maximum amplitude, comparing the amplitudes of eachof said plurality of waveforms to said first maximum amplitude, summingthose waveforms having amplitudes less than said first maximum amplitudeto obtain a resultant stacked waveform.
 10. The method of claim 9,wherein the amplitudes of each of said plurality of waveforms arecompared to said first maximum amplitude over only a portion of eachwaveform.
 11. The method of claim 10, wherein said resultant stackedwaveform comprises an initial formation compressional arrival portion, asubsequent formation shear arrival portion, and a final tube wavearrival portion; andwherein said portion of each of said plurality ofwaveforms, over which the amplitudes are compared to said first maximumamplitude, excludes the portion of each of said plurality of waveformscorresponding to said tube wave arrival portion of said resultantstacked waveform.
 12. The method of claim 9, wherein said processingstep e) further comprises:determining from said stacked waveform thepropagation speed of acoustic energy in said earth formation.
 13. Themethod of claim 12, wherein said acoustic energy comprises compressionalor shear waves.
 14. A method for performing sonic logging while drillinga borehole traversing an earth formation to determine characteristics ofsaid earth formation, the steps of said method comprising:a) drilling aborehole with a drill collar supporting a drill bit; b) transmittingacoustic energy about a given center frequency with a transmittermounted on the drill collar; c) receiving acoustic energy with at leastone receiver mounted on the drill collar; d) providing at least oneoutput related to the received acoustic energy, said at least one outputcomprising a plurality of waveforms; and e) processing said at least oneoutput to determine at least one characteristic of said earth formation,said processing comprising summing said plurality of waveforms to obtaina resultant stacked waveform.